Among the elements listed in the Periodic Table of the Elements, the following are collectively called rare earth metals or rare earth elements: scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. The first practical application of a rare earth metal was the gas mantle invented in the end of the 19th Century wherein cerium was mixed into the luminescence material of a gas lamp, which was five-times brighter than conventional gas lamps. Ever since, the rare earth metals have become indispensable materials in various fields involving optical function applications such as the following: red light fluorescent material in television cathode-ray tubes (europium, yttrium), x-ray CT scintillators in medical diagnosis (gadolinium, praseodymium), light control glasses (neodymium, cerium), solid-state lasers (yttrium, neodymium), electrostatic capacitors (yttrium, gadolinium); optical fiber amplifier (erbium, praseodymium, terbium, dysprosium), and magneto-optical recording disks (terbium, gadolinium). (For a general review, see Yasuo Suzuki, “Kidorui no Hanashi” [The Story of Rare Earth Elements], Shokabo Publishing Co., Ltd. 1998.) On the other hand, Period IV transition metals have been used in a similar manner as fluorescent materials in fluorescent lamps, mercury lamps, and cathode ray tubes. Moreover, because of their absorption properties, the transition metals have been used as various types of inorganic pigments, and they have often been used as magnetic materials as well.
The rare earth metals or/and Period IV transition metals have most often been used for doping in the form of rare earth metal ions or/and Period IV transition metal ions or rare earth metal oxides or/and Period IV transition metal oxides in a host material. Such host materials include glasses, garnet crystals, and transparent ceramic materials (zirconia and the like). However, all such host materials are inorganic, and there have been almost no examples wherein an organic material has been used as a host material.
For example, as an example of a light control optical element, a color filter multicolor correction lens utilizing absorption and the like have been studied.
Japanese Patent Application Laid-open No. H 11-133227 discloses a process for producing color filter wherein an inorganic pigment is formed into a substrate together with low melting glass frit and then enameled. A simpler means is a method wherein a pigment is mixed directly with an organic medium, and Japanese Patent Application Laid-open No. 2003-4930 and Japanese Patent Application Laid-open No. 2004-307853 disclose that a color filter having the required color and transmission characteristics can be obtained through the use of pigment fine particles of 100 nm or less in diameter. Although the pigment fine particles are needed to be dispersed in the organic medium in these means, they can hardly be dispersed without aggregation, which could lower the optical properties such as transmittance, haze, and the like.
As a means of solving the above problems, Japanese Patent Application Laid-open No. 2004-226913 discloses that the uniform mixing of the (meth) acrylic acid salt of a rare earth metal with an acrylic monomer is effective for obtaining color correction and the like and a required color. However, in that method it is necessary to use an acrylic monomer as the organic polymer of the matrix material, and it is impossible to form a composite of colorants in a wide range of organic matrix materials.
In addition, if used as a luminescence material, when doping is performed with a rare earth metal or/and Period IV transition metal at a high concentration in an inorganic material, the rare earth metal or/and Period IV metal ingredients aggregate or markedly get closer to each other. As a result, such a process has a problem that it is difficult to dope at a high concentration because the phenomenon known as “quenching” occurs wherein the luminescence (fluorescence) is diminished, and essentially, only a maximum concentration of about 100 ppm of rare earth metal or/and Period IV transition metal can be attained in practice.
On the other hand, organic materials, organic polymers in particular, have excellent properties in terms of processability, light weight, cost effectiveness and the like, and they have become key materials sustaining modern society. Especially in fields involving optical function applications, which are the fields to which the present invention is closely related, organic polymers are widely used for plastic lenses in applications such as eyeglasses, contact lenses and the like (acrylic resin, polycarbonate, cyclic olefin resin and the like), optical disks (polycarbonate), plastic optical fibers (acrylic resin), and the like (see Fumio Ide, “Oputoerekutoronikusu to Kobunshi Zairyo” [Optoelectronics and Polymer Materials], Kyoritsu Shuppan Co., Ltd. 1995). In addition to the doping into inorganic materials such as glasses, garnet crystals, ceramics and the like, the doping of rare earth metals or/and Period IV transition metals, that are used in various fields involving optical function application, into organic materials, especially organic polymers, makes it possible to bring about novel uses that provide better performance than inorganic materials with respect to processability, light weight, cost effectiveness, and the like.
However, there has been a problem that it has been impossible to manufacture materials wherein an organic polymer as a host material is doped with a rare earth metal or/and Period IV transition metal because rare earth metals or/and Period IV transition metals are hardly dispersible/soluble in an organic medium.
Optical amplifiers are the number one application wherein rare earth metals are used. As our advanced information society has become more widespread, the role of optical communications technology has become more important because the amount of data has become increasingly larger and data processing and transmission rates have become increasingly faster. Therefore, optical communications networks are being built not only as main lines in Japan but also on a global scale. In the 1990s the wavelength division multiplexing (WDM) transmission format wherein a plurality of optical signals with different wavelengths are transmitted simultaneously in a single optical fiber became commercialized, and the construction of large capacity high speed data communications networks was accelerated. One essential basic technology enabling the commercialization of such a WDM transmission format is optical amplification technology. In existing commercial optical amplification technology, an optical signal light in the 1550 nm bandwidth is excited by a semiconductor laser with a wavelength of 980 nm, 1480 nm, and the like. The optical fiber that carries the optical signal light and excitation light is doped with a rare earth metal, and after the rare earth metal is excited by the excitation light, the optical signal light that has become attenuated due to the long distance transmission process is supplemented by superimposing the light emitted by the rare earth metal in the 1550 nm band onto the optical signal light. Erbium is the most well-known rare earth metal that is doped into optical fibers, and the erbium-doped fiber amplifier (EDFA) is widely available for commercial use. In addition to erbium, the development of optical amplifiers using rare earth metals such as praseodymium, thulium and the like in accordance with the optical signal bandwidths used is proceeding vigorously.
In general, rare earth metals are doped in silica glass optical fibers at a concentration of 500 to 1000 ppm. When added at higher concentrations, the rare earth metal elements aggregate and a phenomenon occurs wherein the energy of a rare earth metal atom excited by the excitation light transfers to an adjacent rare earth metal atom before emission of the light corresponding to the optical signal wavelength, and thus the desired emission cannot be obtained. This is called “concentration quenching” and delineates the boundary wherein a rare earth metal can be doped into a silica glass optical fiber. As a result, in practice an optical fiber approximately 100 m long is needed to amplify an optical signal to the practically requisite intensity using excitation light, and this is a factor limiting the miniaturization of optical amplifiers (see Shoichi Sudo, ed., “Erubiumu Tenka Hikari Faiba Zofukuki” [Erbium-added Fiber optic Amplifiers], The Optronics Co., Ltd., 1999, p. 50).
With respect to such optical amplifiers, studies are underway in which, as various types of glass lenses have been replaced by organic polymer molded lenses, conversion of a silica glass matrix material to an organic polymer matrix material is attempted to make low-cost optical amplifiers practical and increase their cost effectiveness, which will be necessary not only for long distance main line fiber optic networks, but also for the massive fiber optic transmission lines of subscriber systems and the like that are becoming more widespread in average households (see Japanese Patent Application Laid-open No. H05-088026, Japanese Patent Application Laid-open No. 2000-208851, U.S. Pat. No. 6,292,292, U.S. Pat. No. 6,538,805, U.S. Pat. No. 6,751,396, Japanese Patent Application Laid-open No. 2000-256251, and Japanese Patent Application Laid-open No. H05-179147).
There is a problem, however, because rare earth metals do not easily dissolve or disperse in an organic medium. As a result, it has been impossible to dope a rare earth metal into an organic polymer matrix material, which provides excellent cost effectiveness in plastic optical fibers and the like, and this makes it difficult to improve greater cost effectiveness in optical transmission networks by the practical application of a low cost optical amplifier.
In general, fluorescent materials contain rare earth metals that can be listed as rare earth metals usable for doping organic polymers. Such fluorescent materials comprise three components, i.e., a host material, an activator, and a co-activator. Crystals of oxides and crystals of ionic compounds are used as the host material (see M. T. Anderson, et al., “Phosphors for Flat Panel Emission Displays,” in B. G. Potter, Jr. et al., eds. Synthesis and Application of Lanthanide-Doped Materials, p. 79, The American Ceramic Society 1996). In other words, the goal has been achieved not by directly doping an organic polymer with a rare earth metal having fluorescence itself as the activator, but by first doping oxide crystals such as yttrium-aluminum-garnet (YAG) and the like with a rare earth metal, and then pulverizing the crystals and mixing them with an organic polymer. However, when such means are used, baking at a high temperature of about 1400° C. is necessary to form the YAG crystals, which increases the processing cost. In addition, the particle size of the pulverized fluorescent materials containing the rare earth metal are generally 1000 nm (1 μm) or more, and when they are dispersed at a high concentration for the purpose of using them in an optical amplifier, transparency is decreased due to optical scattering, and the product can no longer function as an optical waveguide. Thus, there is a concentration limitation below which fluorescent materials, prepared by doping rare earth metals into a host material such as a crystal, can be dispersed in a resin, and it is impossible to achieve both the miniaturization of optical amplifiers by using a high concentration of doping agent and an improvement in cost effectiveness by using an organic material as an optical transmission medium.
As a method of using a host material containing a rare earth metal in the same way as a fluorescent material, a means wherein the rare earth metal is carried by fine particles (Japanese Patent Application Laid-open No. 2003-89756) and a means wherein the rare earth metal is embedded in fine particles by ion implantation (L. H. Slooff, et al., Journal of Applied Physics, Vol. 83, p. 497, 1998) have been proposed, but both involve the problem wherein optical transparency is hindered because of the large size of the fine particles.
On the other hand, as means for doping an organic polymer directly with a rare earth metal, a method for synthesizing an organic/inorganic composite has been proposed wherein (a) an organic coordination compound between a rare earth metal and an organic ligand such as a pyridine, phenanthrolene, quinoline, β-diketone or the like is formed initially, and the rare earth metal is dispersed in an organic polymer thereby; and (b) the rare earth metal is contained in an organic cage complexes, and the inclusion compound is dispersed in an organic polymer; and the like (see L. H. Slooff, et al., Journal of Applied Physics, Vol. 83, p. 497, 1998).
Such means illustrated by (a) and (b) above broaden the type of rare earth metal that can be used and the range of concentration. Moreover, the dispersion phase containing the rare earth metal obtained thereby has molecular order, so even though the dispersion phase may aggregate somewhat, the size can be limited to a range from roughly a few nanometers to about 20 nm, and this enables doping at a high concentration without causing the decrease in transparency that accompanies optical scattering. There is a problem, however, because when these means are used, the excited state energy in the rare earth metal that has been excited by the excitation light transfers to the molecular vibrations of the CH and OH groups in the organic cage complexes or/and organic ligands, that are directly bonded to the rare earth metal, in accordance with the Franck-Condon principle known from quantum mechanics, and the emission process specific to the rare earth metal is inhibited (quenched) thereby (see W. Siebrand, The Journal of Chemical Physics, Vol. 46, p. 440, 1967).
For solving such a problem, a means has been proposed wherein quenching is suppressed by insuring that the excitation energy level of the rare earth metal and the excitation energy level of the organic ligands or organic cage complexes do not overlap by either fluorinating or deuterating the CH groups of the organic ligands of the rare earth metal coordination compound or organic cage complexes (Y. Hasegawa, et al., Chemistry Letters, 1999, p. 35 and Hasegawa “Yuki Baitai Chu de Hikaranai Neodymium o Donoyouni Hikaraseruka?” [How can we make neodymium, which does not emit light in an organic medium, emit light?] Kagaku to Kogyo (Chemistry and Chemical Industry), Vol. 53, page 126, 2000). Such a means is effective in terms of suppressing quenching while enabling a rare earth metal to be dissolved or dispersed in an organic medium at a high concentration. However, the problem remains that the fluorides and deuterides used as a starting material are very expensive, and therefore such a means cannot bring about the effect of improving the cost effectiveness of optical transmission networks that can be expected to accompany the practical application of optical amplifiers having an organic polymer as a matrix material.
A light control optical element can be listed as a second application using a rare earth metal. Light can be sensed by human eyes via various optical elements that control the transmission, refraction, focusing, scattering and the like of light. Many kinds of optical elements can be listed such as the lenses used in eyeglasses, the covers and lighting windows used in various lighting devices, the optical filters used in television receivers, the windows and lenses used in goggles in industrial processes such as welding and in medical therapy, and so on. A variety of optical elements, not only limited to the examples described above, have the important function of controlling the transmission, refraction, focusing, scattering and the like of light in various kinds of optical instruments. Such optical elements generally have a high transparency in the visible wavelength range. However, many other optical elements are also used to control transmission or absorption of both natural and artificial light such as light control lenses, light control glasses, and the like, and, as a group, such elements are called light control optical elements.
Among the lenses for eyeglasses in the examples noted above, lenses for sunglasses, used for the reduction of sickening glare by decreasing light intensity, is the most typical example of a light control optical element. In particular, lenses for sunglasses with a high antiglare effect can be obtained by reducing the amount of light at wavelengths of 400 to 500 nm. As part of the discussion of lenses for eyeglasses, there are persons who have a visual disorder characterized by difficulty in distinguishing colors because they have a congenital visual sensitivity curve that is different from the visual sensitivity curve of normal persons, and for such persons there are corrective lenses that adjust the transmittance value of light to match the visual sensitivity of the person having the visual disorder by selectively amplifying the absorption of light at specific wavelengths. Among the lighting windows or covers listed above, windows and covers with a strong antiglare effect can be obtained for lights having halogen lamps such as those used in automobile headlights and the like by controlling the transmittance of a light at wavelengths of 560 to 600 nm.
Thus, means wherein a material forming an optical element is doped with various light absorbing materials such as pigments and the like are known as means for controlling the transmission or absorption of light of a specific wavelength or wavelength band. Among them, the elements listed above and generally called rare earth metals or rare earth elements or/and Period IV transition metals of vanadium, chromium, manganese, iron, cobalt, nickel and copper, are known to be excellent as doping materials for controlling the transmission or absorption of light of a specific wavelength or waveband in accordance with the use or required purpose, because each element presents a sharp and large absorption spectrum at a particular waveband. With respect to the inventions utilizing the features of these rare earth metals or/and Period IV transition metals, there are many applications such as the following: contrast reinforced glasses and a lens using the same wherein glass is doped with neodymium oxide (Nd2O3) (Japanese Patent Application Laid-open No. H08-301632, U.S. Pat. No. 6,604,824) and windows using the same (U.S. Pat. No. 6,416,867); automobile headlights (U.S. Pat. No. 5,961,208); an optical filter (U.S. Pat. No. 4,106,857) wherein glass is doped with various rare earth elements such as holmium, praseodymium, dysprosium, or the like.
On the other hand, as materials constituting various optical elements that control the transmission, refraction, focusing, and scattering of light and the like, polymer materials with excellent processability, cost effectiveness and light weight have become widespread, and have become indispensable materials technology for modern society, as in the field of light weight lenses for eyeglasses, lenses for optical disk devices and the like. Therefore, in the field of light control optical elements doped with a rare earth metal or/and Period IV transition metal such as those in the preceding paragraph are needed, there is a demand for such elements utilizing polymer materials.
However, rare earth metals or/and Period IV transition metals have the property of dissolving or dispersing very poorly in organic media, and therefore the practical application of light control optical elements having a polymer matrix material that express light transmission and absorption properties specific to rare earth elements has been inhibited thereby.
To resolve such a problem the following means for synthesizing an organic/inorganic composite have been proposed in the past: (a) a coordination compound between a rare earth metal or/and Period IV transition metal and an organic ligand such as a pyridine, phenanthrolene, quinoline, β-diketone or the like is initially formed, and then the rare earth metal or/and Period IV transition metal is dispersed in an organic polymer thereby; (b) a rare earth metal or/and Period IV transition metal is included in an organic cage complexes, and the inclusion compound is dispersed in an organic polymer; (c) a polymer is formed using a polymer synthesizing monomer of a rare earth metal or/and Period IV transition metal; and the like.
When methods (a) and (b) are used, however, absorption in the UV region due to the organic ligands or organic cage complexes tends to increase, and this induces an energy transfer from the organic ligands or organic cage complexes to the matrix material polymer, and UV light degradation of the matrix material polymer by the generation of radicals formed upon cleavage of the molecular chains in the organic ligands or organic cage complexes and the like. In addition, the cost of the starting materials of most such organic ligands or organic cage complexes is high, and the high cost effectiveness that is the feature of a polymer optical element is impaired.
As an example of method (c), a method is known wherein a (meth) acrylic acid rare earth metal salt, hydroxyalkyl(meth)acrylate and phthalic acid, and a monomer copolymerizable with these are mixed, and the mixture is polymerized to produce a polymer (see Japanese Patent Application Laid-open No. 2004-226913). There is a problem, however, because when this method is used, the polymer material that can be used as a matrix is limited to a very small number of polymers such as acrylic resins, styrene resins, and the like, and it is impossible to satisfy the need for polymeric optical elements wherein various polymer materials such as polycarbonate resins, cyclic olefin resins, polyester resins, epoxy resins and the like have been developed as a matrix.
A luminescent device can be listed as a third application using a rare earth metal. Ever since Edison invented the incandescent bulb in the end of the 19th Century, various electric lamps and discharge tubes represented by fluorescent lamps have made major contributions to life in society. In the 1960s, light emitting diodes (LEDs) comprising compound semiconductors such as gallium arsenide and the like became practical, and they became widespread as miniature light emitting elements of infrared light, red light, green light and the like. The optical device of the present invention is a device that converts electric energy to light energy, as typified by various electric bulbs, discharge tubes, LED devices and the like. More specifically, as shown in FIGS. 14(a) to 14(c), the electric energy is converted by a light emitting element to light energy, which in turn is reconverted to light at a wavelength suitable for the application thereof by a fluorescent material containing a rare earth metal that is excited by the light emitted from this light emitting element. Thus, even when the light emitting element itself is one wherein, for example, electric energy is converted to UV light, it becomes possible to obtain light of the various wavelengths peculiar to each of the rare earth metals because this UV light excites the fluorescent material containing the rare earth metal.
Generally speaking, an LED comprising a compound semiconductor such as gallium arsenide, gallium phosphide and the like is known to have a higher luminous efficiency (conversion efficiency from electric energy to light energy) than an incandescent electric bulb or discharge tubes. The emission of red light can be obtained from compound semiconductors such as gallium arsenide, gallium arsenide phosphide and the like; the emission of red to yellow light can be obtained from aluminum indium gallium phosphide; and the emission of green light can be obtained from gallium phosphide. These lights are used in light emitting indicators of various types of electronic equipment, light emitting elements for remote control devices that operate various electronic equipment, LED display modules, and the like. In addition, recently a blue LED has been realized by the invention of a gallium nitride LED, and full color, large screen displays integrating various LEDs emitting the three primary colors of red, green and blue have been put into practical uses. Thus, as LEDs have extensively used from a point light source to a flat panel light source, there arises a vigorous movement in which lights that heretofore have been dependent on incandescent bulbs and discharge tubes are replaced by LEDs. Especially, the use of LEDs, which have a conversion efficiency from electric energy to light energy much higher than that of glass tube incandescent bulbs and discharge tubes, is an effective means to promote energy savings today when the problem of global warming has become severe. In Japan, the technological development of the use of LEDs has been promoted by the “Project for the Development of Compound Semiconductors for High Efficiency Optoelectronic Conversion” (nicknamed: “Light of the 21st Century Project”) established by the Ministry of Economy, Trade and Industry in 1998. In the course of such technical development a white LED has been developed, and LEDs are becoming more widespread as a light source for lamps in place of incandescent bulbs and discharge tubes.
There are three methods for obtaining a white LED: (1) Obtaining white as a mixed color by integrating red, green and blue LEDs; (2) Obtaining white as a mixed color of yellow and blue by generating blue light with a blue LED and simultaneously generating yellow fluorescent light by exciting a fluorescent material with the blue light; and (3) Obtaining white by exciting three types of fluorescent materials that emit red (R), green (G), and blue (B) using a blue LED or an ultraviolet LED, and then combining the three primary colors R, G, and B (Tsunemasa Taguchi, ed. “Hakushoku LED Shomei System no Kokido/Kokoritsu/Chojumyoka Gijutsu” [Technology for High Intensity/High Efficiency/Long Life White LED Lighting Systems] Gijutsu Joho Kyokai, 2003).
If method (1) is used, a drive circuit for each of the three colors must be provided because the operating characteristics of the LEDs corresponding to each of the three colors are different, and this will interfere with miniaturization, less power consumption and the like, and therefore methods (2) and (3) are considered more practical.
Incidentally, it is known that the background of achieving white light that is excellent in intensity and color rendering properties in various types of incandescent and discharge tubes was originated from the use of a rare earth metal as a luminescence material. It is said that the first practical application of a rare earth metal was the gas mantle invented in the end of the 19th Century wherein cerium was mixed into the luminescence material of a gas lamp, which was five-times brighter than conventional gas lamps, and rare earth metals such as cerium (Ce), neodymium (Nd), and europium (Eu) have come to be used in incandescent and discharge tubes. Therefore, even when methods (2) and (3) are used, it is preferable to use these rare earth metals as luminescence materials that are excited by blue light or UV light. However, for using a rare earth metal as a luminescence material, as shown in FIG. 14(c), the general construction of the LED must be sealed with a resin after the light emitting element 31 (in this case, an LED chip) is mounted on a substrate. If the desired rare earth metal 33 such as Ce, Tb, Eu and the like, which are luminescence materials, can be dispersed in the resin as shown in FIG. 14(c), white light can be obtained as a mixed color with the fluorescent light 34 emitted from the rare earth metal 33 that has been excited by the light 32 emitted from the LED. There is a problem, however, because the rare earth metal does not easily dissolve in an organic medium such as the LED sealing resin 35.
To overcome this kind of problem, a fluorescent material containing a rare earth metal has been used in the past in display applications such as television receivers, flat panel displays and the like, because it will disperse the rare earth metal uniformly in an organic medium such as the LED sealing resin 35 and the like. Such fluorescent materials comprise three components, i.e., a host material, an activator, and a co-activator. Crystals of oxides and crystals of ionic compounds are used as the host material (see M. T. Anderson, et al., “Phosphors for Flat Panel Emission Displays,” in B. G. Potter, Jr. et al., eds. Synthesis and Application of Lanthanide-Doped Materials, p. 79, The American Ceramic Society 1996). In other words, the goal has been achieved not by directly doping an LED sealing resin with a rare earth metal itself, which is a luminescence material, but by first doping oxide crystals such as yttrium-aluminum-garnet (YAG) and the like with a rare earth metal, and then pulverizing the crystals and mixing them with a resin.
However, when such a means is used, baking at a high temperature of about 1400° C. is necessary to form the YAG crystals, which increases the processing cost. In addition, the particle size of the pulverized fluorescent material containing the rare earth metal generally has a lower limit ranging from 1000 nm (1 μm) to several hundred nanometers, and when the particles are dispersed at a high concentration, transparency is decreased due to optical scattering that cannot be ignored. Thus, there is a concentration limitation below which fluorescent materials, prepared by doping rare earth metals into a host material such as a crystal, can be dispersed in a resin, and it is impossible to vary the dispersion concentration freely over a wide range.
As means for solving such a problem originating in fluorescent materials that have been used in the past and doping an organic medium directly with a rare earth metal, an organic/inorganic composite synthesis means has been proposed wherein (a) a coordination compound between a rare earth metal and an organic ligand such as a pyridine, phenanthrolene, quinoline, β-diketone or the like is formed, and the rare earth metal is dispersed in an organic medium thereby; and (b) the rare earth metal is included in an organic cage complexes, and the inclusion compound is dispersed in an organic medium; and the like.
Such means illustrated by (a) and (b) above involve broadening the type of rare earth metal and the range of concentration limitations. There is a problem, however, because when these means are used, the excited state energy in the rare earth metal that has been excited by blue light transfers to the molecular vibrations of the CH and OH groups in the organic cage complexes and the organic ligand that is directly bonded to the rare earth metal due to the Franck-Condon principle known in quantum mechanics, and the emission process specific to the rare earth metal is inhibited (quenched) (see W. Siebrand, The Journal of Chemical Physics, Vol. 46, p. 440, 1967, and L. H. Slooff, et al., Journal of Applied Physics, Vol. 83, p. 497, 1998).
For solving such a problem, a means has been proposed wherein quenching is suppressed by insuring that the excitation energy level of the rare earth metal and the excitation energy level of the organic ligands or organic cage complexes do not overlap by either fluorinating or deuterating the CH groups of the organic ligand of the rare earth metal coordination compound or organic cage complexes (Japanese Patent Publication No. 10-36835, Japanese Patent Application Laid-open No. 2000-256251, Y. Hasegawa, et al., Chemistry Letters, 1999, p. 35 and Hasegawa “Yuki Baitai Chu de Hikaranai Neodymium o Donoyoni Hikaraseruka?” [How can we make neodymium, which does not emit light in an organic medium, emit light?] Kagaku to Kogyo (Chemistry and Chemical Industry) Vol. 53, page 126, 2000). Moreover, a luminescent device using a rare earth metal coordination compound obtained thereby has been proposed (Japanese Patent Application Laid-open No. 2003-81986, Japanese Patent Application Laid-open No. 2003-147346). Such a means is effective in terms of suppressing quenching while enabling a rare earth metal to be dissolved or dispersed in an organic medium at a high concentration. However, the problem remains that because the fluorides and deuterides used as a starting material are very expensive, such a means lacks the cost effectiveness required by LED illumination, and this prevents the same from becoming widespread as consumer appliances.